Method for determining filtration properties of rocks

ABSTRACT

This invention relates to the oil and gas industry, more specifically, to the development of heavy oil and asphaltic bitumen deposits. 
     The method of determining the filtration properties of rocks comprises providing contrasting temperature fluid circulation in the well; supplying the fluid is in the quantity required for partial fluid absorption in the annulus; stopping fluid circulation in the well; measuring temperature along the well bore as from circulation stoppage and until the achievement of a thermally stable condition; obtaining a temperature vs time curve along the well bore; determining the filtration properties of rocks using a data averaging model. Fluid viscosity is modified by chemical additives for non-horizontal wells.

This invention relates to the oil and gas industry, more specifically, to the development of heavy oil and asphaltic bitumen deposits.

The permanent growth of hydrocarbon prices and the inevitable depletion of light oil resources have recently caused increasing attention to the development of heavy oil and asphaltic bitumen deposits. Among the existing methods of developing high viscosity hydrocarbon deposits (e.g. mining, solvent injection etc.), thermal methods (hot water injection, thermal-steam well treatment, thermal-steam formation treatment etc.) are known for their high oil recovery and withdrawal rate.

Known is a thermal-steam gravity treatment method (SAGD) which is currently one of the most efficient heavy oil and asphaltic bitumen deposit development methods (Butler R.: “Thermal Recovery of Oil and Bitumen”, Prentice-Hall Inc., New-Jersey, 1991, Butler R., “Horizontal Wells for the Recovery of Oil, Gas and Bitumen”, Petroleum Society of Canadian Institute of Mining, Metallurgy and Petroleum, 1994). This method implies creation of a high-temperature ‘steam chamber’ in the formation by injecting steam into the top horizontal well and recovering oil from the bottom well. In spite of its worldwide use, this deposit development method requires further improvement, i.e. by increasing the oil-to-steam ration and providing steam chamber development control.

One way to increase the efficiency of SAGD is process control and adjustment based on permanent temperature monitoring. This is achieved by installing distributed temperature measurement systems in the wells. One of the main problems related to thermal development methods (e.g. steam assisted gravity drainage) is steam (hot water, steam/gas mixture) breakthrough towards the production well via highly permeable interlayers. This greatly reduces the heat carrier usage efficiency and causes possible loss of downhole equipment. Steam breakthrough response requires repair-and-renewal operations that in turn cause loss of time and possible halting of the project. This problem is especially important for the steam assisted gravity development method due to the small distances (5-10 m) between the production and the injection wells.

Known is a method of active temperature measurements of running wells (RU 2194160). The known invention relates to the geophysical study of running wells and can be used for the determination of annulus fluid flow intervals. The technical result of the known invention is increasing the authenticity and uniqueness of well and annulus fluid flow determination. This is achieved by performing temperature vs time measurements and comparing the resultant temperature vs time profiles during well operation. The temperature vs time profiles are recorded before and after short-term local heating of the casing string within the presumed fluid flow interval. Fluid flow parameters are judged about from temperature growth rate.

Known is a method of determining the permeability of geological areas (RU 2045082). The method comprises creating a pressure pulse in the injection well and performing differential acoustic logging and temperature measurements in several measurement wells. Temperature is measured with centered and non-centered gages. The resultant functions are used to make judgment on the permeability inhomogeneity of the string/cement sheath/formation/well system, and thermometer readings are used to determine the permeability vector direction. Disadvantages of this method are as follows:

-   -   only generalized integral assessment of geological area         permeability is possible;     -   additional multiple measurements (acoustic logging) in several         wells are necessary;     -   the method is not suitable for the characterization of high         viscosity oil and bitumen saturated rocks.

The object of the method suggested herein is to broaden its application area and provide the possibility of quantifying the filtration parameters of rocks along the well bore thereby increasing heat carrier usage efficiency and reducing equipment losses during deposit development.

This object is achieved by using the new sequence of measurements and steps and applying an adequate mathematical model of the process.

Advantages of the method suggested herein are the possibility of characterizing high viscosity oil and bitumen saturated rocks and using standard measurement tools. Moreover, the sequence of steps suggested herein does not interrupt the process flow of thermal development works.

The method of determining the filtration properties of rocks is implemented as follows:

-   -   contrasting temperature fluid circulation is provided in the         well;     -   the fluid is supplied in the quantity required for partial fluid         absorption in the annulus;     -   fluid circulation in the well is stopped;     -   temperature is measured along the well bore as from circulation         stoppage and until the achievement of a thermally stable         condition;     -   a temperature vs time curve along the well bore is obtained;     -   the filtration properties of rocks are determined using a data         averaging model.

The invention will be exemplified below with drawings where

FIG. 1 shows the preliminary heating stage,

FIG. 2 shows the temperature distribution along the well bore after the preliminary heating,

FIG. 3 shows the pressure and temperature profiles during steam injection and

FIG. 4 shows an example of filtration properties distribution as determined from the temperature restoration rate measurements.

The method suggested herein requires distributed temperature measurements over the whole length of the portion of interest at the preliminary heating stage. At that development stage (FIG. 1), a hydrodynamic link is established between the wells by heating the cross borehole space. In the standard steam-assisted gravity development technology, this is achieved by conduction heating of the formation due to steam circulation in both the horizontal wells. The method of determining the filtration properties of rocks suggested herein requires additional works, i.e. partially closing the annulus at the preliminary heating stage to create an excessive pressure inside the well bore. This pressure will force the steam to flow into the formation as long as it is possible. The quantity of steam penetrated into the oil-saturated beds (and hence the quantity of heat) will depend on the local permeability of the formation (FIG. 2). This Figure shows formation portions having different permeabilities: at portion (1) K=3 μm², at portion (2) K=5 μm², at portion (3) K=2 μm², while at other portions K=0.5 μm². As can be seen from FIG. 2, the heat signal received after steam circulation stoppage will be provided by the highly permeable formation portions. Moreover, the temperature restoration rate will depend on the permeabilities of local portions. Thus, the temperature measurement results (provided by the distributed measurement system) after steam circulation stoppage can be used for assessing the permeability profile along the well bore.

To solve the reverse task, this method provides an analytical model satisfying the following properties and having the following boundary conditions:

-   -   one-dimensional frontal cylindrical symmetrical model;     -   in the initial condition, the pore space is fully saturated with         oil/bitumen;     -   the following areas form during steam injection into the         formation (FIG. 3): steam (III), water and hot oil (II) and cold         oil (I);     -   the oil/water boundary is determined as the boundary between the         areas filled with fluids having a significant difference in         viscosity (cold highly viscous oil having viscosity μ₀ and         steam, water and hot formation fluid having average viscosity         μ₁).

The position of the oil/water boundary can be determined using the following equation:

$r_{o} = \sqrt{r_{w}^{2} + \frac{q^{*} \cdot t_{c}}{\pi \cdot \varphi}}$

where

$q^{*} = {c_{q} \cdot {\frac{{k \cdot \Delta}\; P}{\mu_{0}}.}}$

The value of the parameter c_(q)≈0.5÷1.5 can be assessed from numeric simulation/field experiments to allow for the following specific features that can hardly be incorporated into a purely analytical model:

-   -   the temperature and viscosity of oil near the oil/water boundary         differs from those in the formation;     -   actually, there is no clear oil/water boundary (there is a         transition oil/water mixture area).

Thus, the oil/water boundary radius is determined by the following parameters:

-   -   formation permeability (k);     -   pressure upon the formation (ΔP);     -   oil viscosity in the formation (μ₀).

The steam/water boundary position is determined by the energy and weight balance equations and can be found as follows:

$\frac{r_{s}}{t} = \left\{ {{\begin{matrix} 0 & {g_{w} > g_{wm}} \\ \frac{g_{wm} - g_{w}}{2{\pi \cdot \varphi \cdot \rho_{w} \cdot r_{s}}} & {g_{w} \leq g_{wm}} \end{matrix}{r_{s}\left( {t = 0} \right)}} = {{{r_{w}.{Where}}g_{w}} \approx {\frac{2{\pi \cdot \lambda_{fw}}}{c_{w}} \cdot \frac{\ln \left( {1 + \frac{{c_{w} \cdot \Delta}\; T}{L + {\left( {c_{s} - c_{w}} \right) \cdot T_{c}}}} \right)}{\ln \left( \frac{r_{w} + {c_{T} \cdot \sqrt{a \cdot t_{c}}}}{r_{s}(t)} \right)}}}} \right.$

is the steam condensation weight rate,

$g_{wm} = {{\rho_{w} \cdot q^{*}} = {\rho_{w} \cdot c_{q} \cdot \frac{\Delta \; {P \cdot k}}{\mu}}}$

is the maximum condensation rate, ρ_(w) is the density of water, φ is the formation porosity, λ_(fw) is the heat conductivity of the water-saturated reservoir, c_(w) is the heat capacity of water, c_(s) is the heat capacity of steam, α is the thermal diffusivity of the formation, L is the heat of evaporation, t_(c) is the duration of injection and T_(c) is the steam condensation temperature.

The temperature profile at the steam injection stage is as follows:

${T(r)} = \left\{ \begin{matrix} T_{c} & {r \leq r_{s}} \\ {T_{0} + {\left( {T_{c} - T_{0}} \right) \cdot \frac{1 - \left( \frac{r}{r_{T}} \right)^{v}}{1 - \left( \frac{r_{s}}{r_{T}} \right)^{v}}}} & {{r_{s} < r \leq r_{T}},{v = \frac{g_{w} \cdot c_{w}}{2{\pi \cdot \lambda_{fw}}}}} \\ T_{0} & {r_{T} < r} \end{matrix} \right.$

Temperature restoration after steam circulation stoppage can be described with a simple conductive heat exchange model not allowing for phase transitions.

Example of filtration properties (permeability K) distribution assessment based on temperature restoration rate measurements is shown in FIG. 4, the top portion showing the assessment results and the bottom portion showing the simulated values.

Thus, the method of determining the filtration properties of rocks suggested herein allows quantification of the permeability profile along the well bore at an early stage of steam-assisted gravity drainage or another heat-assisted well development method. The resultant permeability profile can be used for the preventive isolation of highly permeable formations before the initiation of the main development stage and allows avoiding steam breakthrough towards the production well. The permeability profile along the whole well bore length is determined by measuring the non-steady-state thermal field with a distributed temperature measurement system. 

1. Method of determining the filtration properties of rocks wherein contrasting temperature fluid circulation is provided in the well; the fluid is supplied in the quantity required for partial fluid absorption in the annulus; fluid circulation in the well is stopped; temperature is measured along the well bore as from circulation stoppage and until the achievement of a thermally stable condition; a temperature vs time curve along the well bore is obtained; the filtration properties of rocks are determined using a data averaging model.
 2. Method of claim 1, wherein fluid viscosity is modified by chemical additives for non-horizontal wells. 